Waters, Brian M.; Lucena, Carlos; Romera, Francisco J.; Jester, Gena G.; Wynn, April N.; Rojas, Carmen L.; Alcántara, Esteban; and Pérez-Vicente, Rafael, "Ethylene involvement in the regulation of the H + -ATPase CsHA1 gene and of the new isolated ferric reductase CsFRO1 and iron transporter CsIRT1 genes in cucumber plants" (2007).

Bicarbonate is considered one of the most important factors causing Fe chlorosis in Strategy I plants, mainly on calcareous soils. Most of its negative effects have been attributed to its capacity to buffer a high pH in soils, which can diminish both Fe solubility and root ferric reductase activity. Besides its pH-mediated effects, previous work has shown that bicarbonate can inhibit the induction of enhanced ferric reductase activity in Fe-deficient Strategy I plants. However, to date it is not known whether bicarbonate affects the upregulation of the ferric reductase gene and other genes involved in Fe acquisition. The objective of this work has been to study the effect of bicarbonate on the expression of several Fe acquisition genes in Arabidopsis (Arabidopsis thaliana L.), pea (Pisum sativum L.), tomato (Lycopersicon esculentum Mill.) and cucumber (Cucumis sativus L.) plants. Genes for ferric reductases AtFRO2, PsFRO1, LeFRO1 and CsFRO1; iron transporters AtITR1, PsRIT1, LeIRT1 and CsIRT1; H+-ATPases CsHA1 and CsHA2; and transcription factors AtFIT and LeFER have been examined. The results showed that bicarbonate could induce Fe chlorosis by inhibiting the expression of the ferric reductase, the iron transporter and the H+-ATPase genes, probably through alteration of the expression of Fe efficiency reactions (FER) (or FER-like) transcription factors.

In a previous work, it was shown that bicarbonate (one of the most important factors causing Fe chlorosis in Strategy I plants) can limit the expression of several genes involved in Fe acquisition. Hypoxia is considered another important factor causing Fe chlorosis, mainly on calcareous soils. However, to date it is not known whether hypoxia aggravates Fe chlorosis by affecting bicarbonate concentration or by specific negative effects on Fe acquisition. Results found in this work show that hypoxia, generated by eliminating the aeration of the nutrient solution, can limit the expression of several Fe acquisition genes in Fe-deficient Arabidopsis, cucumber and pea plants, like the genes for ferric reductases AtFRO2, PsFRO1 and CsFRO1; iron transporters AtIRT1, PsRIT1 and CsIRT1; H(+) -ATPase CsHA1; and transcription factors AtFIT, AtbHLH38, and AtbHLH39. Interestingly, the limitation of the expression of Fe-acquisition genes by hypoxia did not occur in the Arabidopsis ethylene constitutive mutant ctr1, which suggests that the negative effect of hypoxia is related to ethylene, an hormone involved in the upregulation of Fe acquisition genes. As for hypoxia, results obtained by applying bicarbonate to the nutrient solution suggests that ethylene is also involved in its negative effect, since ACC (1-aminocyclopropane-1-carboxylic acid; ethylene precursor) partially reversed the negative effect of bicarbonate on the expression of Fe acquisition genes. Taken together, the results obtained show that hypoxia and bicarbonate could induce Fe chlorosis by limiting the expression of Fe acquisition genes, probably because each factor negatively affects different steps of ethylene synthesis and/or signaling.

The FeEDDHA [iron(3+) ethylenediamine di(o-hydroxyphenylacetic) acid] is one of the most efficient iron chelates employed in the correction of iron clorosis in calcareous soils. FeEDDHA presents different positional isomers: the ortho-ortho (o,o), the ortho-para (o,p), and the para-para (p,p). Of these isomers, the p,p cannot chelate Fe in soil solution in a wide range of pH values, while both o,o and o,p can. The objective of this work was to compare the efficiency of both isomers (o,o and o,p) to provide Fe to two Strategy I plants (tomato and peach) in nutrient solution (pH approximately 6.0), as well as in calcareous soil (pH approximately 8.4; CALCIXEREPT). For this, chelates of both o,o-EDDHA and o,p-EDDHA with 57Fe (a nonradioactive isotope of Fe) were used, where the 57Fe acts as a tracer. The results obtained showed that the o,o isomer is capable of providing sufficient Fe to plants in both nutrient solution and calcareous soil. However, the o,p isomer is capable of providing sufficient Fe to plants in nutrient solution but not in calcareous soil.